I'll share my sidepipe/sound odyssey. Maybe it will help.
During the build, I knew that I didn't want to use the FFR pipes for a few reasons. First, I knew that the packing fairly quickly blew out. Second, I knew that they were raspy to the point of being painful at higher revs and finally, I knew that I wanted to autocross the car and our SCCA region has a fairly strict sound policy. The FFR pipes definitely wouldn't pass sound.
My first iteration was the Classic Chambered Cobra Packs. I got the 32" 2-1/2" ID packed mufflers. These are the quietest mufflers they make. For details on the retrofit, see
here. Even with my crappy welding skills, they came out nicely. The advantage of welding them up yourself is that you can make them fit perfectly, obviating the need for wedges or spacers. I had them ceramic coated at NitroPlate. They look and sound great. They are definitely quieter than the FFR glasspacks, but I wouldn't say they were quiet.
Sadly, they weren't quiet enough for autocross. I was blowing 100dB as measured 50 feet perpendicular from the fastest part of the course where I was close to full throttle. So, back to the drawing board.
After a fairly thorough search of available mufflers, I came across
these. They are 30" mufflers that are packed and contain two comes nose-to-nose. Two things made them attractive - the "mild to moderate" sound description, and the 3.5" ID flow path. So, I ordered a pair (they're not cheap) and also bought a used set of raw steel FFR pipes from the classifieds. I didn't want to cut up my Classic Chambered pipes after all the work I put into getting fit perfectly plus the cost I put into ceramic coating for them. So, I decided to build a second pair of pipes.
The Flowmasters make the job of building the pipes dead-nuts easy. Their ID is exactly the same 3.5" as the OD of the FFR pipe tubing. I simply cut the muffler out of the FFR pipes, leaving the collector and turnout. Both the collector and turnout slide perfectly into the Flowmaster mufflers. All it took was two easy weld beads and the pipes were done.
I have to say that the Flowmasters are very quiet, particularly at idle and cruise. They also have a nice deep rumble with no raspiness. They do, however, get pretty loud at WOT, and I have found that I will still bust sound at autocross occasionally.
So, I added one more modification. I bought a couple of
exhaust turnouts that will slip over 3-1/2" tubing and cut the old turnouts off, leaving about a 2" stub of 3-1/2" tubing over which the new turnouts will slip. I then got a pair of
3-1/2" augers and found that will just a little fiddling and trimming, I could tack weld the augers to the inside of the turnout, and the rest of the auger would slip into the muffler. I riveted the turnouts into place (this is easily reversible and allow me to play with the turnout angle). The result was pretty amazing. The car is almost factory-car quiet - even at WOT. I'm sure there is a performance penalty, but my butt-dyno couldn't detect it.
So, the take-away from this lengthy post is that you can easily make some
very quiet pipes that retain a decent flow path but using the Flowmaster mufflers referenced above with some augers inserted into the aft end. The augers could go in when you make the pipes up and be tack welded to both the muffler and the turnout to prevent rattling. I only left mine removable to give me options to meet sound at autocross.
Tim